The online auction colossus is expanding briskly and eyeing some big opportunities. With the stock down 35% this year, a nice rebound could be in store.

LAST WEEK, EBAY HIT THE BID. In a deal designed to boost the online auction giant's flagging domestic growth rate, eBay agreed to pay $620 million in cash to acquire Shopping.com, which operates an online shopping-comparison service.

It was the latest in a series of moves that have dramatically expanded eBay beyond its original business of providing an online auction market for collectibles. Via its 2002 acquisition of PayPal, for instance, eBay became a key provider of online payment services, competing with the...